The study marks a paradigm shift in our understanding of gamma-ray bursts

The study marks a paradigm shift in our understanding of gamma-ray bursts

Graph of Jet Lorentz prime factors for the 13 GRBs in our sample. These values ​​are obtained by assuming that the energy fraction of the magnetic field is ϵB= 103. Purple bars represent values ​​extracted directly from the data (first category), black bars are upper limits (second category) and blue bars are lower limits (third category). The upper and lower borders are also marked by arrows. The mean value of the initial Jet Lorentz GRB factor is 〈ΓI 〉 ≈ 51 (median 32), although the range range is 1.7 ≲ ΓI ≤ 218 (see Table 3). We indicate that GRB 080607 has the highest value of ΓIIt has a large gap in the X-ray LC between the plateau and the similar phases. Moreover, GRB 171205A which has the lowest Γ valueIAssociated with SN 2017iuk, therefore, both the visual plateau and the self-similar slopes of this eruption are affected by the SN protrusion. credit: Nature Communications (2022). DOI: 10.1038/s41467-022-32881-1

Matter flows in the form of jets are observed in astronomical systems at fast, medium and slow speeds. The fastest planes are highly relativistic, traveling at very close to the speed of light. The origin, as well as many of the aircraft’s characteristics, is uncertain. The jet velocities appear to have a bimodal distribution—some very fast, others slow, with a gap in velocities in between, which has long been a challenge for experts. The Bar-Ilan University researchers re-examined the data and now seem to have solved the mystery.

In many different galaxy systems and extragalaxies, the emission of matter is commonly observed in the form of jets. the Speed Where this happens in a big way. Along with the associated relatively slow aircraft Neutron stars or binary star systems, very fast relativistic jets are seen at speeds very close to the speed of light. The fastest known planes are associated with a phenomenon known as gamma ray bursts.

This phenomenon is characterized by an initial flash of gamma rays lasting a few seconds, during which a strong emission of gamma rays can be seen. Then follows an afterglow that lasts longer than hours, days, or even months. During this era, the emission fades later and is observed as lower wavelengths, X-rays, ultraviolet, optical, infrared, and radio frequencies very late in the process.

Beyond the question of why the jets from these objects speed up, there is a mystery that seems unrelated to what happens during the intermediate period of hundreds to thousands of seconds, during which the emission fades away or remains constant. In some cases, after a few tens of seconds, the X-ray emission decays exponentially, as would be expected from a relativistic explosion colliding with the matter and radiation present in the interstellar space of the galaxy.

However, in about 60% of the cases observed, the visible emission does not fade but rather remains constant. This observation has always been a source of confusion for researchers, and no convincing explanation has been found for it since the discovery of this phenomenon about 18 years ago.

Researchers from Bar-Ilan University’s Department of Physics have now demonstrated that this permanent visible emission is a natural consequence of the jet’s velocity, which is much lower than was usually assumed, and bridges the gap between velocities measured from other sources. In other words, the lower initial jet velocity could explain the lack of decay and the more pronounced and durable emission.

The researchers showed that the previous results, from which high speeds were deduced in these bodies, are not valid in these cases. This changes the old paradigm and proves that planes form in nature at all speeds. The study has been published in the journal Nature Communications.

One of the main open questions in the study of gamma-ray bursts is why in such a large proportion of cases, X-rays, which can be seen for several days, take so long to fade. To answer this question, the researchers set about carefully mapping the data, which was numerous but scattered and “noisy.”

After an exhaustive literature search, they created a high-quality data sample. After examining explanations for the phenomenon in the existing literature, they found that all current models, without exception, make additional assumptions that are not supported by the data. What is more important is that none of the models provided a convincing explanation for the clean data.

So, the researchers went back to the basic model and tried to understand which of the basic assumptions is incorrect. They discovered that changing just one assumption, about the planes’ initial velocity, was enough to explain the data. The researchers went on and examined the data that led other astrophysicists to conclude that the jets must be highly relativistic (ie, traveling very close to the speed of light = extremely fast), and discovered, to their surprise and delight, that none of the jets should be highly relativistic. The arguments found were valid in the cases they studied. From there, they quickly conclude that they are likely headed in the right direction.

Professor Assaf Peer, who led the theoretical part of this research, describes himself as a theoretician who enjoys working with data. “Astrophysical systems are generally very complex, and theoretical models, which are more simple in nature, often miss key points,” he explains. “In many cases, close examination of the data, as we performed here, shows that existing ideas simply don’t work. This is what prompts us to come up with new ideas. Sometimes the simplest, least complex idea is enough.”

Professor Beer’s partners in this research are the first author of the study, Dr Hussein Direli Beg, from the Bar-Ilan Research Group, and Professor Felix Reid, from KTH Royal Institute of Technology in Stockholm. While Beer focused on theory, his collaborators focused on analyzing the data that motivated and supported the theory he proposed.

“It took us some time to develop an understanding, and once I realized there was one standard in the whole that needed to change, the whole thing worked like a puzzle,” says Professor Beer. “So much so that from some point in time, every time we brought up a new potential problem, it was clear to me that the data would be in our favor, and in fact, it was.”

Astrophysical research is, by its very nature, fundamental research. If the researchers are indeed correct, the findings have far-reaching implications that could lead to a paradigm shift in the field, as well as in the understanding of the physical processes that produce jets. It is important to note that the origins of this phenomenon are still not fully known, but it is clearly related to the collapse of a star (or pair of stars) into a black hole. The results of the research are very important in understanding these mechanisms, as well as the type of stars that end their lives in a way that produces powerful gamma rays.

“The scientific research is amazing. New ideas are constantly being born and tested. Since the data is often inconclusive, people often post their ideas and move on,” says Professor Beer. “This was a unique case, and after examining many ideas, I suddenly realized that an explanation could be very simple. After I proposed the explanation, we checked it again and again against the existing data, and it passed test after test. So sometimes the simplest explanation is the most success too.”

more information:
Hüsne Dereli-Bégué et al, Wind environment and Lorentz factors explain dozens of plateau X-ray gamma-ray bursts, Nature Communications (2022). DOI: 10.1038/s41467-022-32881-1

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